Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues as a result of operation with such systems. As one example, as engine load decreases and an opening of an intake throttle is reduced to reduce engine torque output, pumping work of the engine may increase, thereby decreasing fuel economy.
In one example, the issues described above may be addressed by a method, comprising: in response to engine load below a threshold, deactivating all intake valves of an engine cylinder while operating a first exhaust valve coupled to an exhaust gas recirculation (EGR) passage coupled to an intake passage and a second exhaust valve coupled to an exhaust passage at different timings; and routing intake air from the intake passage, through the EGR passage, and into the engine cylinder via the first exhaust valve. In this way, by deactivating the intake valves, intake air may flow in reverse through the EGR passage to the engine cylinder. As one example, the intake air routed into the engine cylinder via the first exhaust valve and EGR passage may be combusted within the engine cylinder and then exhausted to the exhaust passage via the second exhaust valve. This operation may reduce pumping work at lighter engine loads, thereby increasing fuel economy. Additionally, by flowing the intake air in reverse through the EGR passage, the intake air may be warmed via an EGR cooler disposed in the EGR passage. This may increase the pressure of the intake air entering the engine cylinder and further reduce intake pumping, as well as reduce engine emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.